chore(typings): mark underscore-prefix members @internal

I still haven't made a tslint check for this, so new occurrences leaked in.
See https://github.com/angular/angular/issues/4645

Closes #4986

modules/angular2/src/router/router.ts

@@ -478,6 +478,7 @@ export class Router {
 export class RootRouter extends Router {
   /** @internal */
   _location: Location;
+  /** @internal */
   _locationSub: Object;
 
   constructor(registry: RouteRegistry, location: Location, primaryComponent: Type) {